Modern athletic footwear typically include an outsole, midsole, and insole. The midsole is positioned between the ground-contacting outsole and the insole and typically includes one or more foams for attenuating impact forces generated upon the contact of a wearer's shoe on the ground. Foams such as ethelene vinyl acetate (EVA) also have resilience for energy return and are typically included in the midsole of modern athletic footwear. Athletic shoes can also include additional elements in portions of the midsole to impart a desired balance of shock-absorption (cushioning), energy return, and stability to various the user.
For example, U.S. Pat. No. 6,789,332 to Scholz discloses an L-shaped spring element attached to a top support plate. The spring element encompasses a damping element from the side and from below. The L-shaped spring, which enhances energy return, contacts the ground from below and is deflected upward, simultaneously with the damping element positioned above it, on ground contact of the sole. A downward directed restoring force is produced when the foot pushes off the ground. Overpronation or oversupination can be selectively minimized by using a less soft material for the damping material, thereby adding stability to the medial or lateral side respectively.
U.S. Pat. No. 6,216,365 to Cohen discloses a shock-absorbing unit with an effective heel member having a spine portion and elastically deformable flat spring elements branching out from the spine and rising upward to contact a top plate. A second effective member can be positioned with its spine below and contacting the first spine, with elastically deformable flat spring elements branching out and downward to contact a heel plate. A foam pad is positioned between each spine and adjacent plate. The shock-absorbing unit can be designed for specific athletic activities or for therapeutic purposes by modifying the stiffness of one or more spring elements.
U.S. Pat. No. 5,185,943 to Tong et al. discloses a resilient insert member to enhance energy return which can be positioned sandwiched between an outsole and midsole, or encapsulated in the midsole or outsole. The insert member has a central body portion and extensions which extend outwardly and downwardly from the central body portion. The extensions move outward as forces impinge on the sole, and act as a spring to return the sole to its original shape. The insert member may be “tuned” by adding apertures to those extension members located in specific areas of the sole where additional flexibility is desired and building up those extension members where it is desirable for the extension member to be more rigid or less compressible.
U.S. Pat. No. 7,421,805 to Geer discloses a support structure for a shoe that includes an elongated member that extends along the length of the shoe and a number of lateral members that extend perpendicularly from the elongated member. The lateral members may extend in an upward or downward direction or may remain in a horizontal plane.
While athletic shoes are usually designed with stability in mind, some prior art shoes are intentionally designed to promote a lengthwise instability. For example, “EasyTone” is a shoe sold by Reebok International, Ltd. that is intended to create a natural instability and a feeling akin to walking on a sandy beach. This is achieved by inserting balance pods under the heel and forefoot of the shoes. Air travels between the forefoot and heel pod to create soft cushioning. The moving air creates a natural instability and forces your muscles to adapt to the air volume within the pods. The instability encourages toning in the hamstrings, gluteus maximus, and calves.
Another known shoe designed to promote instability is the “MBT” model sold by Masai USA Corp. This shoe uses a PU midsole which has a “balancing area” in the metatarsus region, causing a rocking motion forward and rearward of the balancing area, even when standing. Also included is a soft heel pad that is intended to mimic beach sand. This design is intended to stimulate muscle toning and to burn extra calories in that the muscles tense in reaction to the natural instability. A similar design is described in U.S. Pat. No. 6,421,935 to Bartlett.
U.S. Pat. No. 291,490 to Buch shows leaf springs mounted between an outsole and a midsole.
None of these prior art references provides a shoe sole and a shoe incorporating the shoe sole, which includes a horizontal indentation in a peripheral sidewall of the shoe sole and preferably additional structure that can be tuned to offset a wearer's side-to-side balance and thereby encourage a wearer's conditioning and toning.